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How to Maintain & Repair Trailer Brakes


Trailer brakes are often required on a trailer if a tow vehicle has a gross vehicle weight rating (GVWR) of more than 3,000 lbs. In some states, trailers with a GVWR as low as 1,000 lbs. may be required to have brakes. On trailers with tandem axles, brakes may be required on both axles if the GVWR exceeds 5,000 lbs.

Trailer brakes are essential because the added weight of the trailer may be more than the tow vehicle can safely handle. Having an extra set of brakes on the trailer also improves braking stability and reduces the risk of the trailer jackknifing in a panic stop situation.

Trailer brakes are often ignored until a problem occurs. So it is important to check the brake regularly and make repairs as required.

Trailer brakes are not much different than passenger car and light truck brakes, and they often require the same kind of maintenance and repairs. Corrosion is the main concern with trailers, especially boat trailers. Brakes on boat trailers are often submerged when unloading and loading a boat , making them vulnerable to rust and sticking. Salt water is really hard on the brakes. After three or four seasons of boating, the brakes often have to be rebuilt or replaced. The wheel bearings are also vulnerable to water contamination and corrosion, and may fail if they are not cleaned, repacked with grease and adjusted regularly.


Trailer brakes come in two basic types: electric and surge. Electric brakes are activated by an electrical connection to the tow vehicle's brake pedal, or an adjustable dash-mounted inertia switch in the tow vehicle or on the trailer itself. When the brakes are applied, an electric current proportional to the rate of deceleration energizes a magnet inside each brake. The magnet moves an actuating lever to apply the brakes. When the driver takes his foot off the brake pedal or the vehicle starts to move again, current to the magnet is cut off and the brakes are released. The controller is adjustable to compensate for varying trailer loads.

trailer brakes

With surge brakes, braking is automatic and requires no electrical connection between the tow vehicle and trailer (except for the lights). The surge coupler is mounted on the tongue of the trailer. Inside is a linkage connected to a hydraulic master cylinder. When the tow vehicle applies its brakes, the forward momentum of the trailer pushes on the surge coupler, causing it to slide rearward and apply pressure against the master cylinder piston rod.

The amount of pressure applied to the trailer brakes will be proportional to the force exerted by the trailer tongue against the trailer hitch on the tow vehicle. The greater the rate of deceleration, the greater the hydraulic pressure that is applied to the trailer brakes. After the vehicle has stopped and starts to move again, the forward pull on the surge coupler relieves pressure on the master cylinder and releases the trailer brakes.

As simple as surge brakes are, they cannot tell the difference between normal braking and backing up. Consequently, they either need a free backing mechanism that releases the brakes when backing up, or a reverse solenoid wired to the tow vehicle's backup lights to vent brake pressure when backing up.

Surge brakes are commonly used on boat trailers because electrical brakes do not hold up well in wet environments. They are also used on many rental trailers because they eliminate the need for any electrical hookups other than trailer lights. But in some states, surge brakes are not considered adequate for heavier trailers or commercial applications (like construction equipment). Electric brakes are required, which means the tow vehicle must be equipped with some type of controller to operate the trailer brakes.

Federal law requires all trailers with brakes to have some type of emergency "breakaway" system that will automatically apply the brakes should the trailer accidentally separate from the vehicle that is pulling it. On trailers with electric brakes, this means having an emergency battery backup system that can energize the brakes, and a breakaway switch or pull pin connected to the tow vehicle to activate the brakes if the trailer comes loose. With surge brake systems, a cable or chain connected to the tow vehicle is typically used to apply the trailer brakes in an emergency.

Drum brakes are the most common type of trailer brakes, but many trailers are also equipped with disc brakes. Many drum brakes have a "uniservo" wheel cylinder with only a single piston rather than a "duoservo" brake with two pistons like those on most cars and trucks.

Another difference you will find is that the springs and hardware in boat trailers are often galvanized (zinc plated) rather than painted to improve corrosion resistance. Brake shoes may also be riveted rather than bonded because water-logged brake linings may come loose from their shoes.

Drum brakes can be used with either electric or surge/hydraulic systems, but drums for electric applications must have a flat, machined contact surface for the magnet inside. Replacement drums designed for hydraulic systems only, must not be used on a trailer with electric brakes.

The number of axles and brakes that are required on a trailer will vary depending on the weight capacity of the trailer and government regulations which vary from state to state. As a rule, 7-inch brakes are used on 2,000- to 2,500-lb. capacity axles, 8-1/2- and 10-inch brakes are used on 3,500-lb. axles, and 12-inch brakes are used on six lug, 5,200-lb. axles and eight lug, 6,000- to 8,000-lb. axles.

If only one set of brakes is used on a trailer with tandem axles, they are usually mounted on the front axle.


>For safe driving, the brakes need to be checked to make sure they fully release when the vehicle is in motion, and apply properly and evenly when the vehicle is braking.

The condition of the drums, shoes and shoe return springs and hardware should also be inspected. Badly corroded return springs and shoe hardware should be replaced. Adjusters should turn easily. Shoes or drums that are too thin should also be replaced.

On disc brake systems, inspect rotor thickness, condition and runout. If the rotors are warped, worn too thin or have cracks, they need to be replaced. Check the thickness and condition of the pads, and replace if worn to minimum thickness or if the pad lining is loose or cracked.

On drum and disc brake hydraulic systems, check the wheel cylinders, brake lines and master cylinder for fluid leaks. The condition of the brake fluid should also be inspected for rust, sediment and water contamination. Test strips that react to water contamination or the breakdown of corrosion inhibitors in the fluid can give you a more accurate indication of the fluid's condition than a simple visual inspection.

On electric brake systems, check the condition of the wiring, electrical connectors (especially the main trailer connector), magnets and battery. Make sure the emergency battery is fully charged and securely mounted.

exploded view trailer brakes

To check the operation of the brakes on surge/hydraulic systems, raise the trailer wheels off the ground and make sure the wheels spin freely when spun by hand. Dragging may indicate a frozen wheel cylinder, badly corroded brake parts, a plugged brake line or misadjusted drum brakes. Any looseness or roughness as the wheel spins would tell you the wheel bearings need attention.

Next, use a large screwdriver or prybar to simulate a braking force on the surge coupler. Pushing rearward on the surge coupler should apply both brakes evenly. Maintain pressure for five to 10 seconds to make sure the master cylinder holds and is not leaking internally.

No brakes? Check the fluid level inside the master cylinder. If the level is full, there may be air in the lines, the drums may be out of adjustment or the master cylinder may be defective. If bleeding the lines or adjusting the drums does not fix the problem, remove the brake line from the rear of the master cylinder and apply pressure to the surge coupler. If no fluid comes out, check for obstructions. If none are found, replace the master cylinder.

A short test drive can also be used to confirm proper brake operation and adjustment. If the coupler "clunks" when the brakes are applied, it may be experiencing excessive travel because the drums need to be adjusted to reduce shoe clearance.


On electric brake systems, the operation of the brakes can be checked by activating the emergency stop system to see if it brakes the wheels, or by unplugging the trailer from the vehicle and energizing the trailer brake control circuit with a 12-volt battery to apply the brakes (be sure to use circuit breaker or fuse-protected jumper wires). If the brakes fail to apply, check wiring continuity, the trailer circuit breaker or fuse, ground connections and magnets.

Even if the brakes apply normally, the only way to know for sure if they are adjusted properly is to test drive the trailer behind a vehicle.


On electrical systems with adjustable gain, the amount of braking can be adjusted up or down to match road conditions, load and driver preference. Heavier loads require more braking force than light loads. Rough, loose road surfaces or wet roads require less braking force to prevent wheel lock up.

Before the gain adjustment can be properly set, the brakes need to be warmed up by driving a short distance (1/4 mile) at 25 to 45 mph with the brakes lightly applied or by making a series of stops. Warm brakes are more aggressive than cold ones so it is important to make sure the brakes are warm before any gain adjustments are made. Also, the trailer should be loaded normally to simulate real-world driving conditions because the amount of gain that is needed will depend on the load.

Tow the trailer at approximately 25 mph and brake normally on a dry, paved road surface. Gradually increase the gain (brake force) until the brakes just start to lock up, then back off the gain until the trailer comes to a safe, steady stop without the brakes locking up. If the control also has an adjustable delay setting, this too should be fine tuned so the tow vehicle and trailer work in harmony when the brakes are applied.

If the trailer brakes seem to be operating normally in the service bay, but the trailer does not brake properly when it is being driven, the problem may be in the vehicle controller. Some controllers have built-in diagnostic LEDs that illuminate to indicate various operating conditions and faults.


If you suspect a vehicle controller problem, connect a voltmeter or 12 volt test light to the vehicle's trailer connector and check for maximum voltage output when the controller manual over-ride button is pressed. If you do not see maximum voltage at the trailer connector within three or four seconds, check the voltage output at the controller. If the controller is putting out the specified voltage, the problem is in the wiring between the controller and trailer connector. If you do not get the proper output from the controller, the controller may be bad.

A controller may not work properly if it is not picking up a signal from the brake light switch when the brakes are applied. You should see a voltage output from the controller when the brake pedal is depressed with the key on. If there is no output, check the connection between the brake light switch and controller. Also, make sure the controller is getting voltage from the battery and is properly grounded.

Inertial-style controllers must also be installed in the proper position (level or within a specified number of degrees) to sense changes in momentum properly. If someone has remounted or moved the controller, it may have upset the brake adjustment.

Electrical braking problems may also occur if the wires used to carry the current are too small for the amp load in the trailer circuit. Wires should be 14 gauge or larger for both the positive wire from the controller and negative ground wires back to the tow vehicle. Wiring connections should be soldered or joined with compression (crimped) connectors, not pinch-style connectors. Also, the trailer frame and brake cluster backing plate should not be used for ground connections. Brake magnets in the trailer should also be wired parallel (not in series) for optimum performance.


The wheel bearings will usually regular maintenance, especially on boat trailers because of water contamination. Every year, the bearings should be removed, cleaned, inspected, repacked with fresh wheel bearing grease, and readjusted. Some experts caution against using pressurized hub bearing caps because too much pressure may force grease past the seals and contaminate the brake linings.Wheel bearing preload must be set to manufacturer's specifications.

The brakes themselves should be inspected at the same time the wheel bearings are serviced, and any worn, damaged or badly corroded parts replaced. Drum brakes should be adjusted so the linings just touch the drum lightly.

The points where the shoes contact the backing plates on drum brakes should also be lubricated with a moly-type brake grease, along with all hinge points, adjusters and actuator levers.

Brake lines should be bled to remove air if a line has been opened to replace a component. Replacing the fluid annually for preventive maintenance can also help inhibit internal corrosion and prolong the life of the master cylinder and wheel cylinders or calipers, especially on boat trailers.

Related articles:
Doing A Complete Brake Job

Choosing & Installing a Trailer Hitch

Trailer Towing Safety Tips (California DMV)
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